1. Field of the Invention
The present invention relates to an apparatus and method for processing substrates. Specifically, the invention relates to a method for depositing a conformal layer of material on a substrate in an ionized metal plasma process.
2. Background of the Related Art
Sub-quarter micron multi-level metallization represents one of the key technologies for the next generation of ultra large-scale integration (ULSI) for integrated circuits (IC). In the fabrication of semiconductor and other electronic devices, directionality of particles being deposited on a substrate is important to improve in filling of electric features. As circuit densities increase, the widths of vias, contacts and other features, as well as the dielectric materials between them, decrease to 0.25 xcexcm or less, whereas the thickness of the dielectric layer remains substantially constant. Thus, the aspect ratios for the features, i.e., the ratio of the depth to the minimum lateral dimension, increases, thereby pushing the aspect ratios of the contacts and vias to 5:1 and above. As the dimensions of the features decrease, it becomes even more important to get directionality in order to achieve conformal coverage of the feature sidewalls and bottoms.
Conventionally, physical vapor deposition (PVD) systems have been used to deposit materials in device features formed on a substrate. PVD systems are well known in the field of semiconductor processing for forming metal films. Generally, a power supply connected to a processing chamber creates an electrical potential between a target and a substrate support member within the chamber and generates a plasma of a processing gas in the region between the target and substrate support member. Ions from the plasma bombard the negatively biased target and sputter material from the target which then deposits onto a substrate positioned on the substrate support member. However, while such processes have achieved good results for lower aspect ratios, conformal coverage becomes difficult to achieve with increasing aspect ratios. In particular, it has been shown that coverage of the bottoms of the vias decreases with increasing aspect ratios.
One process capable of providing greater directionality to particles is ionized metal plasma-physical vapor deposition (IMP-PVD), also known as high density physical vapor deposition (HDP-PVD). Initially, a plasma is generated by introducing a gas, such as helium or argon, into the chamber and then coupling energy into the chamber via a biased target to ionize the gas. A coil positioned proximate the processing region of the chamber produces an electromagnetic field which induces currents in the plasma resulting in an inductively-coupled medium/high density plasma between a target and a susceptor on which a substrate is placed for processing. The ions and electrons in the plasma are accelerated toward the target by a bias applied to the target causing the sputtering of material from the target by momentum transfer. A portion of the sputtered metal flux is then ionized by the plasma to produce metal ions in the case where the target comprises a metal. An electric field due to an applied or self-bias, develops in the boundary layer, or sheath, between the plasma and the substrate that accelerates the metal ions towards the substrate in a vector parallel to the electric field and perpendicular to the substrate surface. The bias energy is preferably controlled by the application of power, such as RF or DC power, to the susceptor to attract the sputtered target ions in a highly directionalized manner to the surface of the substrate to fill the features formed on the substrate.
One of the problems with HDP-PVD processes is the inability to achieve conformal step coverage in the increasingly smaller device features. Conformal coverage of the bottom and sidewalls of the features is needed to optimize subsequent processes such as electroplating. Electroplating requires conformal barrier and seed layers within the device features in order to ensure uniform filling of the feature. While conventional HDP-PVD achieves good bottom coverage due to the directionality of the ions provided by the bias on the substrate, the sidewall coverage can be less than conformal. This result is caused in part by the induced high directionality of ions towards the bottom of the features with little directionality toward the sidewalls.
The effects of a bias on a substrate can be described with reference to FIGS. 1-2 which illustrate the direction of metal ions 14 entering a via 16 formed on a substrate 10. FIG. 1 illustrates a PVD processing environment wherein no bias is supplied to the substrate 10. As a result, the directionality of the ions 14 is determined primarily by the ejection profile of material (usually atoms) from the target and by the inelastic collisions with other particles in the chamber, such as Ar ions which are provided in a plasma. The angular distribution 22 of the ions 14 in FIG. 1 typically results in little deposition on the bottom 18 of the via 16 due to a large proportion of the ions 14 striking the substrate 10 at oblique angles.
FIG. 2 illustrates the processing environment in a HDP-PVD process wherein the angular distribution of the ions 14 is influenced by the electrical field E due to the applied or self-bias at the surface of the substrate. The electric field E is oriented perpendicular to the substrate 10 and the positively charged ions 14 travel along a trajectory parallel to the electric field E toward the bottom 18 of the via 16. The angular distribution 24 of the ions 14 in FIG. 2 is typically results in moderate to low deposition on the sidewalls 20 and high to moderate deposition on the bottom 18 than is possible without the bias. As compared to the angular distribution 22 of FIG. 1, the distribution 24 exhibits a tighter pattern indicating more directionality parallel to the electric field E.
One of the reasons for poor sidewall coverage of device features in HDP-PVD processes is the orientation of the electric field E shown in FIG. 2. The electric field E extends between the substrate and a chamber component that provides a return path for the RF currents supplied to the support member during processing. Typically, the chamber component is an annular conductive member, such as a metal process shield, disposed proximate to the substrate. Additionally, the chamber component is grounded to support the flow of current to ground.
A schematic representation of a chamber 25 having a substrate support 26, a coil 30 and grounded conductive process shield 27 is shown in FIG. 3. A substrate 28 is disposed on the substrate support 26 for processing and a plasma 29 is maintained in the chamber 25 near the substrate 28. Due to the annular shape of the process shield 27, the field lines of the electric field E between the plasma 29 and the substrate 28 are uniformly distributed with a substantial vertical component relative to the substrate 28. As a result, during processing, ions experience a force due to the electric field E causing the ions to be accelerated down toward the bottoms of the device features formed in the substrate 28 with little direction toward the sidewalls of the features.
Therefore, there is a need to provide a technique for depositing a layer conformally over the surface of features, particularly sub-half micron and higher aspect ratio features.
The present invention generally provides an apparatus and method for depositing a conformal layer on device features in a plasma chamber using sputtered ionized material. In one embodiment, a chamber having a target, a substrate support member and a magnetic field generator to ionize the target material is provided. The target comprises the material to be sputtered by a plasma formed adjacent to the target during processing. The magnetic field generator may be one or more single-turn or multi-turn coils adapted to ionize the sputtered material. The invention provides methods and apparatus adapted to affect the angular distribution of ions present in the chamber.
In one aspect of the invention, a method of modulating the orientation of an electric field between the support member and one or more current return plates is provided. The electric field is generated by applying a reference signal to the support member and providing a current return path through one or more of the return plates. A phase shifted reference signal provided to one or more of the return plates determines the return path of the currents. Preferably, the electric field is rotated about a central axis of the processing chamber at a desired frequency. Additionally, the electric field strength may be modulated by varying the signal power supplied to the support member. A reference signal is provided to the support member to supply a bias to a substrate disposed thereon.
In another aspect of the invention, an apparatus is provided having one or more return plates disposed in the chamber. The return plates and the support member are each coupled to a signal source adapted to produce a reference signal and a phase shifted reference signal. Preferably, the apparatus includes a phase shift network disposed between the signal source and the return plates wherein the phase shift network is adapted to further split an input signal.